These teachings relate generally to photoluminescent compositions or structures, and more particularly, to the preparation and use of photoluminescent compositions or structures that utilize phosphorescent material(s) having low rare earth mineral content in combination with fluorescent material(s) to generate long-persistent luminescence, that is, for extended durations at desired emissive wavelengths.
Luminescence is the emission of electronic radiation from a molecule in its electronically excited state. The source, generally referred to as the excitation source, used to bring a molecule to its electronically excited state, indicates the type of luminescence produced. For example, the emission of electronic radiation from a molecule that is excited, i.e. in an electronically excited state, by electromagnetic radiation is referred to as “photoluminescence.” The persistence of luminescence, also referred to as afterglow, depends on a number of factors, such as, but not limited to, the electronic structure, including the absorption and emission spectra of the emission centers, depths of trapping centers, and possible existence of various quenching centers or defects, the radiation spectrum of the excitation source, as well as environmental conditions, especially that of ambient temperature.
In the past metal sulfide pigments were utilized in an attempt to arrive at phosphor materials that exhibited long-persistent luminescence, e.g., U.S. Pat. Nos. 3,595,804 and 6,207,077, in which the metal is substituted and activation occurs via various elemental activators, co-activators, or compensators. Examples of common activators that were used include copper, aluminum, silver, gold, manganese, gallium, indium, scandium, lead, cerium, terbium, europium, gadolinium, samarium, praseodymium, and other rare-earth elements and halogens. These activators are believed to enter the crystal lattice of the host material and are responsible for imparting luminescent properties to such host material.
As a result, various metal sulfide pigments, such as, CaS:Bi,Tm, which emits violet blue light; CaSrS:Bi,Tm, which emits blue light; ZnS:Cu,Co, which emits green light; and ZnCdS:Cu,Co, which emits yellow or orange light, were exploited in an effort to generate long-persistent luminescence under various conditions. Unfortunately, such metal sulfide photoluminescent phosphors were shown to be environmentally unstable and exhibited photolytic instability. In addition, these phosphors displayed rather quick decaying luminescence, thus indicative of only modest persistence, i.e., less than a half hour after the cessation of the excitation source. Consequently, such metal sulfide pigments have been limited to indoor applications, as well as applications that do not require luminescence for prolonged periods of time.
As an alternative to metal sulfide pigments, a second generation of persistent phosphors, that is, alkaline earth aluminates were developed. The luminescence of these second generation persistent phosphors is found to be much brighter and much longer in duration than that of the metal sulfide pigments. Among them, the green aluminate phosphor, SrAl2O4:Eu2+,Dy3+, is the most commonly utilized due to its emission wavelength, 520 nm, which correlates to the peak of sensitivity of human photopic vision. However, the green aluminate phosphor contains a substantial amount of rare earth minerals, which require excavation and purification that create pollution and environmental problems. It is this reliance on substantial amounts of rare earth minerals that results in the cost of such phosphor to be highly unstable and dependent on uncontrollable factors. In addition, as a result of its host material, such phosphor is shown to be unstable in the presence of water, as well as in high humidity environments.
Given the current phosphors utilized to generate long-persistent luminescence, there is, therefore, a need to utilize more cost efficient phosphors to generate long-persistent luminescence without requiring high amounts of rare earth minerals. It is also desirable, to combine such phosphors with fluorescent materials either in a composition or each in individual layers of a structure such that a wide color gamut of long-persistent luminescence may be realized. Furthermore, it is also desirable, for such phosphors to have the ability to produce long-persistent luminescence under various conditions, such as in the presence of water, without sacrificing emission intensity.